Electric control system



Jan. 29, 1.946. E MoYER I 2,393,701

ELECTRIC CONTROL SYSTEMS Original Filed May 2, 1942 Inventor; Elmo E.M05 r,

H is Attorney Patented Jan. 29, 1946 g1 ELECTRIC CONTROL SYSTEM Elmo E.Moyer, Scotia, N. Y., assignor to General Electric Company, acorporation of New York Original application May 2, 1942, Serial No.

441,450, now Patent No. 2,387,601, dated Octo- Divided and thisapplication March 3, 1944, Serial No. 524,902

3 Claims. (Cl. 250-27) My invention relates to an electric controlsystem and more particularly to a control systerm for dynamo-electricmachines, and to electric valve control circuits. This application is adivision of my application Serial No. 441,450, filed May 2, 1942, PatentNo. 2,387,601, dated October 23, 1945 and assigned to the same assigneeas the present application. Electric valve equipment has been founddesirable in the control of dynamo-electric machines, such as motors andgenerators, due to the great precision of control provided thereby. Inaccordance with the teachings of my invention described hereinafter, Iprovide anew and on the motor during starting and running conditions.

For a better understanding'oi my invention, reference may be had to thesingle'flgure of the drawing taken in connection with the accompanyingdescription and its scope will be pointed out in the appended claims.The single figure of the drawing diagrammatically illustrates anembodiment of my invention as applied to an electric valve controlsystem for an alternating current synchronous motor which drives a. loaddevice, such as a fan, through a magnetic clutch.

Referring now to the single figure of the drawing, I have thereillustrated my invention as applied to a system for controlling adynamo-electric machine which may be an alternating current synchronousmotor'l comprising armature windings .2 and a field winding 3. ,Iprovide starting means for the synchronous motor I.

improved electric valve or electric discharge con- This starting meansmay be any of the control circuit wherein a dynamo-electric machine,ventional arrangements, now well known in the such as an alternatingcurrent synchronous moart, for starting a synchronous motor, and fortor, is controlled. the purposes of illustration I have chosen to rep-It is an object of my invention to provide a resent the starting meansas comprising a source new and imp oved e ectri C tro system. of reducedvoltage 4' which may be connected to It is another object of myinvention to prothe armature windings 2 by means of a circuit vide a newand improved electric valve control controlling means or switch 5. Afterthe speed system for dynamo-electric machines. of the synchronous motorI has attained a pre- It is a further object of my invention toprodetermined value, the switch 5 is opened and vide a new and improvedelectric control system v switch 6 is closed, thereby connecting thearfor a dynamo-electric machine such as an altermature windings 2 to analternating current cirnating current motor of the synchronous type.cuit I of normal operating voltage.

It is a still further object of my invention to The rotating member ofthe synchronous moprovide a new and improved electric valve contor I isconnected to load means which may BT01 y t for n a ter ating currentmotor oi. comprise a magnetic clutch 8 having relatively the synchronoustype and which provides .,for rotatable members 9 and ill, the former ofwhich control of the load imposed on the motor during may be connectedto the rotating member of the the starting operation. synchronous motorI through a shaft H, and

It is a still further object of my invention to the latter of which maybe connected to a load provide a new and improved electric valvecondevice, such as a fan l2, throug a shaft l3. T trol system for analternating current synchromagnetic clutch 8 is provided with a controlnous motor which drives a load device, through winding M, theenergization of which determeans such as magnetic clutch, and in whichthe mines the relative rotation of the members 9 and output speed of theload device iscontrolled. I0 and, hence, controls the speed of the fanl2.

Briefly statedin the illustrated embodiment 0 In other words, theenergization of the control of my invention I provide a new and improvedwinding [4 determines the magnetic flux linking electric valve systemfor an alternating current the members 9 and Ill, and thereby determinessynchronous motor which drives a load device the slip between these twomembers. through a magnetic clutch. The magnetic cou- I provide currentsupply means l5 for enerpling of the clutch is controlled in response togizing the control winding ll of the magnetic an operating condition ofa driven load device. clutch 8. This current supply means may com- Theelectric valve control system not only mainprise an electric valverectifier which is enertains an operating condition, such as the speedgized from a suitable alternating current cirof the load device at asubstantially constant .cuit l6 and which is connected to the controlvalue, but also serves to limit the load imposed 0 winding l4 through acircuit l1. Although not limited thereto, I have chosen to represent therectifier of the current supply means l5 as con- .stituting a quarterphase rectifying system in which a transformer I8 is provided withprimary and secondary windings arranged in the conventional Scott orT-connection. Primary winding means l9 may be connected to thealternating current circuit l6 through a circuit controlling means, suchas a contactor 20 having contacts 21-23, inclusive. Secondary windings24 are connected to circuit [1 through electric valve means 25-28,inclusive. If desired, switches 29 may be connected between the contacts2i-23 of contactor 20 and the alternating current circuit 16.

Electric valve means 25-28, inclusive, are preferably of the typeemploying an ionizable medium, such as a gas or a vapor capable oisupporting an arc discharge, and each comprises an anode 30, a cathode3|, and a control member such as a control grid 32' which determines theamount of current conducted thereby. Cathode heating elements 33, ifemployed, may be energized from a suitable source of current 34. In someapplications it may be desirable to employ electric valves having shieldgrids 35 which may be connected directly to the cathodes 3|. Transientabsorbing capacitances 36 may be connected in the manner illustrated toprevent the im ression of extraneous transient voltages on control grids32.

I also employ an excitation circuit 31 for supplying to the controlgrids 32 of electric valve means 25-28 an alternating component of gridpotential. This portion of the control circuit for the electric valvemeans 25-28 may include a transformer 38 having primary windings 39 andsecondary windings 40, which are also connected in a Scott orT-connection and are phased with respect to the associated anodevoltages to have a 120 electrical degree lagging relationship. Thewindings 45 are connected to the respective associated grids 32 throughcurrent limiting resistances 4|. I also provide suitable voltagelimiting means, such as a resistance 42, connected across the outputcircuit of the rectifier and connected across the control winding I4.This resistance may be of the type having a negative impedance-currentcharacteristic such as that disclosed and claimed in United StatesLetters Patent No. 1,822,742, granted September 8 1931, upon anapplication of Karl B. McEachron. Resistance 42 serves to preventexcessive voltage transients across winding l4 in the event the electricvalve means fail to conduct current at a time when considerable energyis stored in winding [4.

The output of the current supply means l or the conductivity of theelectric valve means 25-25 is controlled by means of a control circuitcomprising conductors 43 and 44. The magnitude of the voltage providedby this control circuit con trols the phase position of the resultantvoltage impressed on control grids 32 of electric valve means 25-28,and, hence, controls the time during the respective positive half cyclesof applied anode-cathode voltage at which the electric valve means beginto conduct current.

I provide a direct current circuit including a positive conductor 45 anda negative conductor 46. This circuit may be energized from thealternating current circuit [6 through a transformer 4! comprisingprimary windings 48 and secondary windings 45, the latter of which maybe provided with an electrical neutral connection 50 which is connectedto the negative conductor 46. Diametric windings of the secondarywindings 45 constitute, with unidirectional conducting devices orelectric valves 5| and 52, a pair of bi-phase rectifiers for energizingthe direct current circuit comprising conductors 45 and 46. Smoothingreactances 53 and 54 are connected between the electric valves 5| and 52and the positive conductor 45. It will be observed that by virtue ofthis connection continuity oi. energization of the direct currentcircuit is provided, even though one rectifier circuit fails.

Control means are provided for controlling the energization of thecontrol winding 14 to maintain a predetermined operating condition ofthe load device at a. predetermined constant value. In the embodiment ofmy invention illustrated, I provide means for maintaining an operatingcondition such as the speed of the fan i2 con-- stant. In addition, Ialso provide armature current limiting means or armature current presetmeans for controlling the maximum value of armature current of the motorI during the starting and running operations.

The control means which controls the voltage diiierence appearingbetween conductors 43 and 44 and which controls the current transmittedby electric valve means 25-28 may comprise an electric discharge device55 provided with a grid 56. The discharge device 55, which is preferablyoi the high vacuum type, may be connected in series relation with aresistance 51 and a constant voltage reference means, such as a glowdischarge valve 58, which when in a conducting condition maintainsacross its terminals a substantially constant voltage. For the purposeof assuring continuity of service, a second electric discharge device 59having a grid 65 may be connected in parallel with the discharge device55, and a second glow discharge valve 5| may be connected in parallelwith the glow discharge valve 58. Electric discharge devices 55 and 53conduct variable amounts of current through resistance 51, therebycontrolling the potential of conductor 44 relative to conductor 43 andconsequently controlling the voltage impressed on control grids 32 ofelectric valve means 25-23, inclusive.

Control electric discharge devices 52 and 53 provided with grids 64 maybe employed for controlling the potential of grids 56 and 50 of electricdischarge devices 55 and 55. The anodecathode circuits of the controldischarge devices l2 and 53 are in parallel and are connected in seriesrelation with a resistance 55 across conductors 45 and 45. The controldischarge devices 52 and 63 transmit variable amounts of current throughresistance and consequently control the potential impressed on grids 56and 60 of discharge devices 55 and 59. The potential of the cathodes oithe electric discharge devices 55 and 58 may be maintained at asubstantially constant value, below the potential of conductor 45, bymeans of glow discharge valves 66 and 51, which also may be connected inparallel in order to assure continuity 01' service.

As a means for deriving a control voltage which isresponsive to apredetermined operating condition of the (an 12, such as the speedthereof, I provide a tachometer generator 68 having an armature 59 and afield winding 10. The armature 65 of the tachometer generator Bl isconnected to control electric discharge devices 52 and 53 throughcircuit H and through apparatus described hereinafter. The negativeterminal of circuit H is connected to grids 64 of control electricdischarge devices 62 and 63 through a resistonce 12 and current limitingresistances T3 and 14. I also provide a filter or damping means whichmay comprise a capacitance I5 which is connected between the common'iimcture-oi resistances I2, 13, and I4, and the cathodes of dischargedevices 62 and 53. In this manner, the

rate of variation of the potential impressed oncontrol grids 64 ofdischarge devices 62 and 63 is controlled to prevent hunting orovershooting of the fan speed.

In order to effect a rapid build-down of the current in the controlwinding I4 when the system indicates a rapid deenergization of controlwinding I4, I provide means for controlling the electric valves 25-28,inclusive, to act as an inverter, wherein the decrement of theelectromagnetic nergy stored in control winding I4 is transferred toalternating current circuit I6. This feature of control is disclosed andbroadly claimed in United States Letters Patent No. 2,239,429, grantedApril 22, 1941, upon my application and which is assigned to theassignee of the present application. More particularly, I provideelectric discharge devices 16 and 11 which may be connected in parallel.It is to be understod that a single discharge device would perform thefunction satisfactorily and that the two discharge devices are employedin order to assure continuity of service. Each of the discharge devicesI6 and I1 is provided with a control grid 18 which is energized by avoltage which is responsive to the magnitude andrpolarity of theunidirectional voltage appearing across conductors 43 and 44. Thiscontrol voltage may be obtained by means of a voltage divider comprisingserially connected resistances I9, 88 and 8| which are connected betweenthe positive conductor 45 of the direct current circuit and the upperterminal of the glow discharge valves 58 and 6|. justable'contactor 82which controls the time at which the electric discharge devices 16 andI1 initiate the inversion operation.

'Armature current limit means or armature current preset means 86 areprovided to limit the effective range of operation of electric dischargedevices 55 and 59. The armature current preset means may compriseelectric discharge devices 81 and 88 having grids 89. The dischargedevices 81 and 88 are employed as a pair to assure continuity ofservice, and only one device is necessary in order to carry out thefunction. The discharge devices 81 and 88 are also connected in parallelwith the electric discharge devices 55 and 59 and thereby limit theanode-cathode voltage thereof by conducting difierent amounts ofcurrent.

The potential impressed on grids 89 of electric discharge devices 81 and88 is controlled by means of a control circuit including conductors 98and 9I. The grids 89 are also connected to grids 56 and 68 of electricdischarge devices 55 and 59 through an anti-hunting or damping circuitincluding a capacitance 92 and resistances 93 and 94. When the voltageof the circuit comprising conductors 98 and 9| rises to a sumcientlypositive value, the electric discharge devices 81 and 98 conduct anincreased amount of current, thereby limiting the range of operationResistance 88 is provided with an adof discharge devices 55 and 59, anddischarge devices 55 and 59 are thereby rendered conducting to an extentwhich lowers the potential impressed on conductor 44 relative toconductor 43 to limit the conductivity of electric valve means andfilter circuit comprising parallel connected resistances 95 and 96 maybe'connected across conductors 98 and 9I'=to limit the efiective rate ofchange of the potential difference therebetween.

I provide, in the armature current preset means 86, means for impressingon the circuit including conductors 98 and 9| a unidirectional voltagethe magnitude of which varies in response to the current of thepolyphase alternating current circuit I. This latter means comprises apair of biphase rectifiers energized in accordance with the currents ofdifferent phases of the polyphase circuit 1 and each comprises atransformer 91 having a primary winding 98 which may be connected inseries relation with the phase conductor, and a secondary winding 99provided with an electrical neutral connection I88. The electricalneutral connections I88 are joined together andare connected to thenegative conductor 9|. Each of the biphase rectifiers comprises a pairof unidirectional conducting paths which may be furnished by electricvalves I 8| and I82; and current Iimitinginductances I83 and I84 areconnected between the positive conductor 98 and valves IN and I82.

I Voltage limiting means, such as resistances I85 and I86, may beconnected across secondary windings 99 of the transformers, and may beof a material such as that described in the abovementioned McEachronpatent. I also connect across the secondary windings 99 voltagecontrolling or adjusting means which may comprise voltage dividersincluding resistances I81 having mechanically coupled adjustablecontacts I88, so that upon adjustment of one means-the other isautomatically adjusted to the same value thereby assuring the desiredcurrent limit or current preset if one of the biphase rectifiers fails.

I provide means for presetting the initial current transmitted by thecurrent supply means I 5, thereby controlling the rate of application ofload to the synchronous motor I during the starting and runningoperation thereof. In this manner, the load is applied gradually to themotor. One way in which this gradual application of load to motor I maybe effected is by limiting the maximum conductivity of the electricvalve means 25-28 and, hence, limiting the maximum initial energizationof control winding I4. More particularly, the initial energization ofthe winding I4 may be limited by biasing the control electric dischargedevices to a predetermined initial degree of conductivity.

One way in which the desired initial presetting of the system may beprovided is by interlocking the contactor 28 with the portion of thecontrol system including the electric discharge devices. provided withcontacts I89 and H8 which are connected between the positive terminal orconductor 45 and the conductor 98 through a resistance III. In thismanner the potential of conductor 98 is initially established therebypreestablishing the degree of conductivity of electric discharge devices55 and 59 and, hence, preestablishing the initial current tobe'conducted by electric valve means 25-28, inclusive.

Contact II8 of contactor 28 is interlocked through the fieldenergization initiating system for the synchronous motor I, so that thecurrent conducted by supply means I5 is maintained ata'predeterminedvalue until the synchronous mo- For example, thecontactor 28 may be tor l attains the desired predetermined speed atwhich the field winding 3 may be energized. I provide means, such as afrequency responsive relay H2 having an actuating coil H3,and contactsH4 and H5. Actuating coil H3 may be energized from a circuit H6 which isenergized in response to the frequency of the currents induced in thefield winding 3 of the synchronous motor I, so that relay H2 isenergized to-.close contacts H when the speed attains a predeterminedvalue. Upon closure of contacts H5, unidirectional current istransmitted to the field winding 3, thereby causing the motor I to stepinto synchronism. Contacts H4 are connected in circuit with contacts H0of contactor so that the initial conductivity of the electric valvemeans -28 is maintained until the field winding 3 is energized, or untilthe speed of the motor I has attained a value which permits synchronousoperation thereof.

Actuating coil H1 of contactor 20 is arranged to be energized from asuitable source of control current H8 through apparatus describedimmediately hereinafter. In order to prevent connection of the electricvalve means 25-23, inclusive, to the alternating current circuit I6 fora predetermined time to permit the cathodes thereof to assume a safeoperating temperature, I employ a time delay relay H9 having contactsI20 connected in series relation with the actuating coil H1. Contactor20 is also provided with sealing-in contacts I2Ia. An auxiliary startingswitch I2I may be connected in series relation with the actuating coilH1, but is not effective of course until the actuating coil H1 has beenenergized by apparatus described hereinafter. A stop circuit controllingmeans, such as a stop switch I22, is connected in circuit with coil H1and this means is, of course, effective upon actuation at all times todisconnect the current supply means I5 from the alternating currentcircuit I6.

I provide control circuits I23 and I24 which may be physically displacedfrom each other and which may constitute remote control stations forpie-establishing the initial load imposed on synchronous motor I and forpre-setting the speed or controlling the speed of the fan I2. CircuitsI23 and I24 each comprise a voltage divider ineluding. a resistance I25which is connected, through switch I21 in parallel with valves 58 andGI, and through switch I26 the contact I30 of which is connected tocircuit II to control electric discharge devices 62 and 63. Moreparticularly, the resistances I25 are connected to the upper or positiveterminals of glow discharge devices 58 and BI through an adjustablemeans, such as an adjustable resistance I28, the function of which willbe explained hereinafter. Other terminals of resistance I25 areconnected to the negative terminals of the glow discharge devices 58 andSI through a conductor I29. Each of the control circuits I23 and I24 isprovided with an adjustable contact I30 the position of which determinesthe speed of the fan I2 and which consequently, in order to obtain thiseffect, determines the conductivity of the electric discharge devices55, 59, 62 and 63. As a means for presetting these discharge devices sothat the system is in a condition which will not impose a severestarting load on the synchronous motor I, the contacts I30 are providedwith interlocking contacts I3I which prevent energization of theactuating coil H1 of contactor 20, unless the electric discharge devices55, 59, 62 and Glare set to establish the proper starting condition.Adjustable contacts I30 are connected to the positive conductor ofcircuit 1I through a conductor I32. A resistance I33 is connectedbetween conductor I32 and the negative conductor 46 of the directcurrent circuit. Resistance I28 determines the maximum speed of the fanI2 when contact arm I30 is moved to the switch end I21 of resistanceI25.

The operation of the embodiment of my invention shown in the singlefigure of the drawing will be explained by considering the system firstduring the starting operation. Switch I2I is closed and switches 29 areclosed, the contactor initiating the energization of the time delayrelay H9 which closes its contacts I20 a predetermined interval of timeafter the energization thereof. If it be assumed that it is desired tocontrol the system by means of circuit I23, its switches I26 and I21 areclosed. Energization of the actuating coil I I1 of contactor I20 willnot be effected until the contact I30 associated with the voltagedivider I25 is adjusted to the predetermined lower position, effectingclosure of contacts I9I and initially biasing the electric dischargecontrol system to a condition which assures a predetermined initialenergization of the control winding I 4, thereby assuring that a heavystarting load will not be imposed on the motor I. Upon closing contactsI3I, actuating coil I I1 is energized effecting operation of contactor20 which, through its contacts 2I-23, connects primary windings I9 oftransformer I0 to the alternating current circuit I6. Upon suchoperation, the sealing-in contacts I2 Ia are closed so that theadjustable contact I30 in control circuit I23 may be subsequently movedto a position along resistance I25 which pre-establishes the desiredspeed of the fan I2. However, the speed responsive part of the controlsystem does not become effective until operation of apparatus describedimmediately hereinafter.

It will be noted that by virtue of contacts I09 of contactor 20 duringthe time prior to the op eration of contactor 20, the electric dischargedevices 55 and 59 have been maintained at a predetermined degree ofconductivity, as determined by the ratio of resistances III and 95, tolower the potential impressed on control grids 32 of electric valvemeans 25-28, inclusive, to a value which limits the amount of currenttransmitted to the control winding H.

Of course, the synchronous motor has been started by first connectingthe source of reduced voltage 4 to the armature windings 2 throughswitch 5, and when the speed attains a sufficiently high value, switch 5is opened and switch 6 is closed thereby connecting the armature winding2 to the alternating current circuit 1 of normal operating voltage. Whenthe speed ofsynchronous motor I attains a predetermined valueestablished by the adjustment of the frequency responsive relay H2, thisrelay will operate energizing the field winding 3 with direct currentand effecting opening of its interlocking contacts H4. These lattercontacts maintain the potential of conductor at a value which preventsthe increase of current to the control winding 4 until the motor I hasattained a sufficient speed, or until the motor I is operating insynchronism.

It will be noted that during the starting operation, means are providedfor maintaining the load on the electric valve means to a value belowthat which would impose a severe load or overload on the motor I.

During normal operation of the system, variable amounts ofunidirectional current are transmitted to the control winding I4 of themagnetic clutch 8 by electric valve means 25-28 to maintain the speed ofthe fan I2 at a precisely constant value. The amount of currenttransmitted by the electric valve means 25-28 is determined by themagnitude of the unidirectional voltage impressed on the control grids82 by means of the circuit including conductors 48 and 44. Of course,the resultant voltage impressed on control grids 82 comprises analternatin component of voltage provided by secondary windings 48 and aunidirectional component of voltage of either positive or negative valuedetermined by the operation of the electric discharge devices 85, 88 orI8, I1. As the magnitude of the voltage produced by control circuitincluding conductors 48 and 44 becomes less negative or more positive,thev electric valve means 25-28 conduct agreater amount of current, andconversely as the voltage becomes more negative the electric valve meansconduct a smaller amount of current.

The operation or the tachometer generator 88 and the control circuitwhich energizes the circuit including conductors 43 and 44 ,will now beconsidered. During normal ranges of operation, the potential diflerenceappearing between conductors 48 and 44 changes in response to the speedof the fan I2 so that uponan increasein speed above a predeterminedvalue or speed level, the output or the electricvalve means -28 willtend to decrease, thereby permitting a greater slip between the members8 and I8 of the magnetic clutch 8. On the other hand, upon a decreaseinspeed below the preset value the output current of the electric valvemeans 25-28 will be increased, thereby decreasing the slip between. themembers 8 and I8 of. the clutch 8 and tending to restore the speed ofthe tan I2 to the desired value. The output voltage of the tachometergenerator 88, effective through circuit II and which is proportional tothe speed or the fan I2, is measured against a predetermined portion oithe standard or reference voltage which is determined or establishedbythe glow discharge valves 88 and 8I. The amount oi, this standardvoltage which is preselected by means of the voltage divider includingresistance I28 in circuit I28 and resistor I28 determines the voltagewhich the tachometer generator 88 must attain in order to control theoutput of the electric valve means 28-28. The dirierence between theoutput of the tachometer generator 88 and the predetermined percentage01' the glow discharge valve reference voltage is amplified by thecontrol electric discharge devices 82 and 83, and the amplified voltageor signal is applied to the grids of discharge devices 88 and 88.Discharge devices 85 and 88 may be considered to constitute thevariableimpedance arm of a Wheatstone bridge, the other three armsotwhlch are resistance 81, resistance I8, resistance 88, and resistance8|. When the voltage applied to the grids 88 and 88 of discharge devices55 and 88 is such that the voltage drop across 81 is equal to thatacross resistance I8 and resistance 88, then the output voltage from thebridge is zero, and so is the unidirectional component of voltagebetween conductors 48 and 44 so that the output current of the electricvalve means 28-28 will be that corresponding to a point within, thecontrol range. I: grids 58 and 5 88 of electric discharge devices 85 and88 are made less negative, or positive, with respect to their associatedcathodes, the potential of conductor 44 will be depressed or lowered,and will be negative in polarity with respect to conductor 43, becausedischarge devices 55 and 58 will pass more current and the voltage dropacross resistance 81 will be greater than that across resistance I8 andresistance 88.

. When the potential of conductor 44 is negative with respect toconductor 43, the control point of the grids 32 of electric valve means25-28 are lagging in phase by a greater amount and the output from theelectric valve means is decreased. Conversely, when the grids 58 and 88of discharge devices 55 and 58 are made less positive or more negative,the potential of conductor 44 will rise because discharge devices 55 and58 will pass less current, and the voltage drop across resistance .51will be less than that across resistance 18 and above the presetvaluedetermined by the adjustment of contact I38 in control circuit I23, the

grids of the discharge devices 62 and 83 become less positive or morenegative, so that these tubes transmitless current and the lowerterminal of resistance 85 rises in potential so that the grids ofdischarge devices 55 and 58 become less negative, or more positive, anddischarge devices 55 and 58 transmit more current so that conductor 44is depressed or'lowered in potential, thereby .phasing back the gridpotentials of the electric valve means25-28 and decreasing theexcitation current transmitted to the control winding I4, so that thespeed of the fan I2 is caused-to decrease until an equilibrium conditionis reached.

The current limit or preset means 88 serves to limit the armaturecurrent of the synchronous motor 2. The current limit control isobtained by causing a unidirectional voltage to appear betweenconductors 88 and 8| proportional to the armature current of motor I,and to cause electric discharge devices 81 and 88 to shunt electricdischarge devices 85 and 58 and, hence, take control away from thelatter discharge devices when the armature current attains the valuepreset by the current limiting means. The voltage appearing betweenconductors 88 and 8I is compared against the reference voltage producedby glow discharge valves 58 and GI, and when the armature current 01'the synchronous motor tends to exceed a predetermined value this voltagebetween these conductors will be approximately equal to the standardvoltage provided by the glow discharge values. 'Under this condition,the grids 88 of the discharge devices 81 and 88 will tend to become lessnegative, or more positive,

, so that these discharge devices transmit a greater amount of current,thereby making the voltage drop across resistance 51 greater than thatacross resistance 18 and resistance 88, and consequently depressing orlowering the potential of conductor 44. In this manner, theconductivity. of the electric valve means 25-28 is decreased, causingvoltage appearing between conductors 98 and BI is just sufiicient tocontrol discharge devices 81 and 88 to furnish exactly the desiredcoupling excitation to the magnetic clutch 8. When the discharge devices81 and 88 are coming into operation, discharge devices 55 and 59 arebeing unloaded because of a consequent fall in speed as indicated by thetachometer generator output voltage.

In view of the above description of the system, it will be apparent thatat a predetermined value of armature current the control of the systemis transferred from the voltage responsive position of the circuit tothe armature current preset means 86, thereby limiting the load imposedon the motor I. It will thus be appreciated that electric dischargedevices 55 and 59, through the control operations of discharge devices62, 63, and 81, 88 are selectively responsive to the speed controllingoperation and the armature current preset means 86 or the armaturecurrent of the synchronous motor I.

The operation of the current preset means 86 will now be considered indetail when the system is initiated in its operation, that is, whenstarting the synchronous motor from standstill. During this startingoperation, sudden application of load to the synchronous motor I isprevented. Adjustment of resistance I88 determines at which value ofarmature current the transfer from armature current control to speedcontrol becomes effective. The take-over control involving resistors IIIand 95 and contacts I88 of device 20 causes a potential to appearbetween conductors 90 and 9|, and appears in effect as though there werean excessive or high armature winding current in motor I. This voltageoccurs prior to any such overload current so that discharge devices 81and 88 control electric valve means 25- 28 to maintain a low degree ofconductivity thereof. When the contactor 20 is energized, its contactI89 releases this take-over control, provided, of course, that thefrequency responsive relay I I2 is closed, and the voltage appearingbetween conductors 8i! and SI which does not in eflect represent anoverload armature current condition but may be considered asrepresentative of a fictitious condition, is allowed to decrementallydecrease until it is sustained by a true or actual armature currentcondition obtained from the armature current indicating or controllingmeans comprising the biphase rectifiers of the armature current presetmeans 86. Adjustment of resistance I08 sets the value of the voltagebetween conductors 90 and SI in a sense determines at What value ofarmature current the control shifts into either current limit operationor speed control operation. The ratio of the potential betweenconductors 90 and SI to the armature current is adjusted by means ofvoltage dividers comprising resistances III! and I08 in the two biphaserectifiers. The voltage impressed across conductors 88 and SI thenrepresents the armature current of the motor I.

In the event the synchronous motor I falls out of step, relay II2 willbe opened, and if contactor 20 is closed will apply a voltage to thecircuit including conductors 90 and BI which causes electric dischargedevices 81 and 88 to reduce the coupling of the clutch 8 or to maintainit at a relatively low value, so that when synchronous motorre-synchronizes and when relay I I2 closes, the coupling of the clutch 8will be applied gradually and not suddenly as the voltage betweenconductors 90 and BI decreases. When this voltage decreasessnfllciently, the control of the System will be taken over by the loadresponsive portion of the control system.

Because the control system could lower the potential of conductor 44sufficiently so that the grids of the electric valve means 25-28 wouldbe effective to render these electric valve means completelynonconducting when the speed responsive system indicates a substantialdeenergization of the control winding H, the range of the biasingpotential impressed on grids 32, that is, the negative valve, is limitedto control electric valve means 25-28 to act as an inverter, therebytransferring the decrement of electromagnetic energy of winding I4 tothe alternating current circuit I6. The electric discharge devices I6and 11 establish a limit to the negativ potential to which the conductor44 may be lowered with respect to conductor 43. This controlling effectis obtained when conductor 44 tends to assume a potential below thatlevel determined by the adjustable contact 82 of resistance 88. When thepotential difference between conductors 43 and II attains apredeterminedvalue, electric discharge devices I6 and TI conduct current to maintainth potential of conductor 44 at this predetermined maximum negativevalue in spite of the tendency of electric discharge devices 55 and 58or discharge devices 81 and 88 to depress the potential of conductor Hto a value more negative.

While I have shown and described my invention as applied to a particularsystem of connections and as embodying various devices diagrammaticallyshown, it will be obvious to those skilled in the art that changes andmodifications may be made without departing from my invention, and I,therefore, aim in the appended claims to cover all such changes andmodifications as fall within the true spirit and scope or my invmtion.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In combination, a control circuit, a direct current circuit, meansfor energizing said direct current circuit, means for controlling thepotential of said control circuit and comprising a pair of electricdischarge devices each having a pair of electrodes and energized fromsaid direct current circuit, means for controlling the conductivity ofone of said discharge devices in response to a predetermined controllinginfluence, means for controlling the conductivity of the other dischargedevice in response to the magnitude of the voltage of said controlcircuit, a third electric discharge device having a pair of electrodesdirectly connected to corresponding electrodes of said one dischargedevice for establishing a shunt current path to said one electricdischarge device, and means for controlling the conductivity of saidthird discharge device to render inefiective said one discharge device.

2. In combination, a control circuit, a direct current circuit, meansfor energizing said direct current circuit, means for controlling thevoltage of said control circuit comprising a pair of serially connectedelectric discharge devices each having a pair of electrodes andenergized from said direct current circuit, means for controlling theconductivity of one of said discharge devices in response to apredetermined controlling influence, means for controlling theconductivity of the other discharge device in response to the magnitudeof the voltage impressed on said control circuit, an alternating currentcircuit, a third electric discharge device having a pair of electrodesdirectly connected to corresponding electrodes of said one dischargedevice for establishing a shunt current path to said one electricdischarge device, and means responsive to the current of saidalternating current circuit for controlling the conductivity of saidthrid electric discharge device to modify the effect of said onedischarge device.

3. In combination, a control circuit, a direct current circuit, meansfor energizing said direct current circuit, means for controlling thevoltage of said control circuit comprising a pair of serially connectedelectric discharge devices each having a pair of electrodes andenergized from said direct current circuit, means for controlling theconductivity of one of said discharge devices in response to apredetermined controlling influence, means for controlling theconductivity of the other discharge device in response to the magnitudeof the voltage supplied to said control circuit, an electric circuit, athirdelectric discharge device having a pair of electrodes directlyconnected to corresponding electrodes of said one discharge device forestablishing a shunt cutthird electric discharge device to modify theefv fact 01' said one discharge device.

ELMO E. MOYER.

